Killed samples of a monoclonal bacterial strain and an axenic alga were offered in different proportions to Tisbe holothuriae, T. battagliai and T. furcata. Incorporation of carbon ranged from 0.05 and 0.39 µg C (adult male)-1 d-1. Using a radiolabelling technique, the ratio in which the two food types were assimilated was compared to the ratio in which they were offered. Neither T. holothuriae nor its sibling species fed selectively under our conditions, but T. furcata showed a clear preference for bacteria. These findings constitute the first evidence for intrageneric dietary differentiation in harpacticoids. The results of the feeding experiments parallel observed differences in mandible structure. They render all the more acute the problem of resource partitioning between T. holothuriae and T. battagliai.